The invention relates to the method and apparatus for speed measurement according to the laser-Doppler-principle.
With speed measurement (special-filter-principle), a screen is projected from equidistant lines of light onto the measured object or into the measuring medium. With the movement of the surface of the measuring object or medium perpendicular to the grid lines, the reflected light experiences an intensity modulation which comprises a spectral maximum which is directly proportional to the movement speed.
Since the projected light pattern with its grid constants represents the material measure, the measuring accuracy of the whole system is directly dependent on the accuracy of the screen. With laser-Doppler speed measurement, this screen is produced within the measuring volume by the superimposition of two coherent part beams which arise, by way of beam splitting, from a laser beam or are produced by two synchronised lasers.
The screen constant determining the accuracy is dependent on two essential parameters, that is
the superposition angle of both part beams PA1 the wavelength of the laser light. PA1 k.sub..lambda. is the relative change of .lambda. PA1 To is the reference temperature.
For the beam splitting which is chiefly applied, there are various known solutions. One lies in directing by way of laser light through a semipermeable planar mirror at 45.degree. to the beam path, a first part beam onto the measured object and via a second planar mirror directing the transmitted part beam likewise on the measured object. With the application of a so-called beam splitting cube with which the splitter surface is arranged at an angle of 45.degree. to the beam path, a part beam is directed onto the measured object via a planar mirror. By way of turning the mentioned beam splitting cube about 45.degree. with respect to the beam path, there is also the possibility of producing two part beams without an additional mirror in that the splitter surface is arranged at an oblique angle to the beam path. Finally it is also known to produce beam splitting with an optical grid or with holographic-optical elements.
Both the first mentioned systems have several disadvantages. The use of several components is disadvantageous with regard to the vibrational stability. Furthermore one must take account of the high cost of angular adjustment for the separated reflecting surfaces. The part beams have differing optical wavelengths which particularly with lasers of a small coherence length, for example with diode lasers, can lead to a detraction of the superposition contrast.
With the use of a beam splitting cube with the latter mentioned version, it is difficult to achieve an optimal beam splitter ratio. Slight errors in the planicity of the beam splitter surface have a considerable effect. Since the angle of impingrnent is relatively small, with an unclearly defined index transition, this has a negative effect on the cementing surface of the beam splitting cube. The polarisation dependency of the reflection/transmission ratio of the splitter surface, with obligue impinging angles, cannot be neglected.
The latter mentioned variation, that is the use of grids or holographic elements requires a relatively high expense. Moreover there are problems with dampness as well as the danger of ageing with holograms on reproduction base material. The optical efficiency is relatively small.